1. Field
The embodiments discussed herein are related to an electromechanical transducer film and a method for manufacturing the electromechanical transducer film.
2. Description of the Related Art
References related to embodiments discussed herein are as follows.
In non-patent document 1 (K. D. Budd, S. K. Dey, D. A. Payne, British Ceramic Society Proceedings 36, 107, 1985), there is introduced a technology related to forming a thin film of a metal complex oxide by using a sol-gel method.
In non-patent document 2 (A. Kumar, G. M. Whitesides, Applied Physics Letters 63, 2002, 1993), it is described that alkanethiol can be formed as a self-assembled monolayer (SAM) on an Au film. It is also described that a SAM pattern can be transferred with a micro-contact printing method using this phenomenon and used in a subsequent process such as etching.
In patent document 1 (Japanese National Publication of International Patent Application No. 2002-543429), there is introduced a technology of forming biological molecules and a cell array by forming an alkanethiol, having the ends of the SAM molecules chemically modified, on a reaction substrate surface and partially removing the SAM by UV irradiation.
In patent document 2 (Japanese Laid-Open Patent Publication No. 04-168277), it is described that a complex oxide thick film is formed on a substrate using an sol-gel method and applied to an actuator or a sensor for a spatial light modulator or inkjet head of a printer.
In patent document 3 (Japanese Laid-Open Patent Publication No. 2005-310962), it is described that a laminated structure 1 is constituted of a wettability change layer 2 which contains a material which varies a critical surface tension by application of energy and has at least two regions where the critical surface tensions are different, consisting of the high surface energy part 3 of a higher critical surface tension and the low surface energy part 4 of a lower critical surface tension, a conductive layer 5 formed in the region of the high surface energy part 3 based on the wettability change layer 2, and a semiconductor layer 6 provided so as to come into contact with at least the region of the low surface energy part 4 based on the wettability change layer 2. Thus, the laminated structure 1 is formed to have a fine conductive layer which can be simply manufactured by the method at a low cost and a high material use efficiency as the printing method, and the semiconductor layer 6 of a high mobility.
In patent document 4 (Japanese Laid-Open Patent Publication No. 2003-297825), there is disclosed a method of fabricating a ferroelectric thin film by using two or more inkjet heads, different sol-gel solutions and evenly mixing the different sol-gel solutions on a plane.
In patent document 5 (International Publication Pamphlet No. WO 2003/098714), there is disclosed a typical configuration of a liquid jet head using a piezoelectric thin film.
In patent document 6 (Japanese Patent No. 4269172), there is disclosed a method for manufacturing a ferroelectric film by having an inkjet head eject a sol-gel solution which is prepared for the inkjet head.
[Conventional Method for Forming Individual Piezoelectric Elements]
A piezoelectric film formed of a metal complex oxide is deposited on a lower electrode by using known depositing techniques such as a vacuum deposition method (e.g., sputtering method, MO-CVD method (chemical vapor deposition using metal oxide), an ion-plating method), sol-gel method, a hydrothermal synthesis method, an AD (aerosol deposition) method, an MOD (metal organic decomposition) method. Then, after forming an upper electrode on the piezoelectric film, patterning is performed on the upper electrode by photolithography and etching. Then, in a similar manner, patterning is performed on the piezoelectric film and the lower electrode. Thereby, individualization of piezoelectric elements is achieved.
A metal complex oxide (particularly, PZT (lead zirconate titanate)) is a material that cannot be easily dry-etched. A Si (silicon) semiconductor device can be easily etched by using a RIE (Reactive Ion Etching) method. However, because the material used for forming the Si semiconductor device increases the plasma energy of ions, the material used for forming the Si semiconductor device requires a special type of RIE using ICP (Inductively Coupled Plasma), ECR (Electron Cyclotron Resonance) plasma, and helicon plasma together. This results in an increase of manufacturing cost. In addition, it is difficult for a substrate to improve a selection ratio with respect to the lower electrode. Particularly, an uneven etching rate is a critical problem for a substrate having a large area.
Although the aforementioned etching process can be omitted if a PZT film (being difficult to etch) can be formed only at a desired part of a lower electrode beforehand, attempts for forming the PZT film in such manner have hardly been made.
[Conventional Method for Forming Individual PZT Films]
Thermal decomposition method: Because PZT can be selectively grown on a Ti metal by performing a thermal decomposition method, a PZT film can be formed only at a desired part by performing a patterning process on a Ti electrode. In order to obtain a PZT film having sufficient pressure-resistance by using the thermal decomposition method, it is preferable to form a relatively thick PZT film having a thickness of 5 μm or more. If the thickness of the PZT film is less than 5 μm, the PZT film can be easily destroyed by applying an electric field thereto. Thus, the PZT film cannot be formed as a thin film having a desired thickness. In a case of forming a device (element) on a Si substrate, the Si substrate will require protection because thermal decomposition is performed on the Si substrate in a strong alkaline solution.Vacuum decomposition method: In manufacturing an organic EL (electroluminescent) device, a luminescent layer is patterned by using a shadow mask. The shadow mask is typically formed of a stainless material. Meanwhile, a PZT film is to be deposited in a state where the temperature of the substrate is 500-600° C. Because complex oxide is required to be crystallized on a substrate for generating a piezoelectric characteristic, the substrate is heated to 500-600° C., so that such crystallized complex oxide film can be obtained. Thus, due to a thermal expansion difference between a Si substrate and a stainless material, satisfactory masking cannot be achieved in a case where a stainless shadow mask is used. In addition, the use of a disposable shadow mask is unrealistic. Particularly, in a case where a MO-CVD method or a sputtering method is used on a substrate, the deposited film tends to come around to the other side or another part of the substrate. Therefore, the MO-CVD method and the sputtering method are particularly unsuitable.AD method: With the AD method, a resist pattern is formed on a substrate beforehand by photolithography, so that a PZT film can be formed in an area(s) of the substrate with no resist pattern (resist film). The same as the above-described thermal decomposition method, the AD method is suitable for fabricating a thick film but not suitable for fabricating a thin film having a thickness equal to or less than 5 μm. Because PZT film is also deposited on the resist film, a liftoff process is to be performed after a portion of the deposited PZT film is removed by a grinding process. It is, however, difficult to perform grinding evenly on a large area. Further, because the resist film has no heat resistance, deposition with the AD method is performed at room temperature.Then, the deposited film is converted to a film having a piezoelectric characteristic after being subject to a post-anneal process.Sol-gel method: In a case of using a sol-gel method where a coating of ink is applied to a surface of a substrate formed of platinum by performing an inkjet technique, the ink dedicated for the inkjet method tends to wet and spread on the surface of the platinum substrate. This is because the ink dedicated for the inkjet technique has a low viscosity. Further, in a case where a large amount of ink is applied at a single time in the sol-gel method, a “coffee stain phenomenon” occurs in which a center portion of the ink coating is thin whereas the edges of the ink coating are thick (protrude). Therefore, the sol-gel method is unsuitable for forming a thick film.